I've not found other guides out there about this online, but I do know that there's ways to date a dupont lighter. The real question is what time frame each of these elements indicates. The first thing is that older duponts tend to say "de Paris" rather than just "Paris". They also tend to have shorter 6-digit serial numbers rather than the 7-digit ones found on newer models. Older lighters will almost always have the older large square dupont emblem with "ST Dupont" spelled out at the bottom and the circle with wings above it.

Older Ligne 1 lighters made before 1984 have an automatic gas shutoff when you open the flint chamber. This is supposedly not the case on lighters made after 1984, but I'd like some more confirmation on this. I've personally handled 4 ligne 1 lighters and all of them had the automatic shutoff, although it was somewhat busted on one of them, only shutting off if you hold down the flint chamber release button, likely just from wear / age, but that lighter otherwise functions fine so it wasn't worth it to repair it.